In internal combustion engines which operate according to the diesel process, an SCR catalyst is often provided in the exhaust gas system in order to meet environmental regulations. In order that the SCR catalyst can convert the NOx compounds contained in the exhaust gas to water and atmospheric nitrogen, a liquid urea-water solution (reducing agent) has to be injected into the exhaust tract upstream of the SCR catalyst. To this end, a dosing system comprising a tank, a pump and a dosing module, which operates similarly to the injector of a fuel injection system, is used. The pump is also referred to as the delivery module.
It is the task of the delivery module or the pump to draw urea-water solution from a tank and to build up a sufficient pressure on the pressure side so that the liquid urea-water solution is finely atomized as soon as the dosing module opens in a demand-controlled manner. Just as the delivery module, the injector is connected to a control device of the internal combustion engine and is opened by said device in accordance with the demand and closed again by the same. The same process also applies to the operation of the delivery pump. Because urea-water solution has the property of freezing at low temperatures and thereby increasing the volume thereof by approximately 11%, measures must be taken to prevent damage to the dosing system due to freezing urea-water solution.
For this reason, it is known from the German patent application DE 10 2004 054 238 to aerate the lines carrying the urea-water solution. To this end, the pump is designed having a reversible delivery direction, or a valve for reversing the delivery direction of the pump is provided.
It is known from the German patent application DE 10 2009 029 408 to integrate a 4/2-way valve into the dosing system. In a first switching position of the 4/2-way valve, the pump delivers reducing agent from the tank to the dosing module. When the internal combustion engine is to be shut down, the 4/2-way valve is brought into the second switching position; thus enabling the pump of the delivery module to deliver liquid reducing agent from the dosing module to the tank and thereby ventilate parts of the dosing system. This assumes that the dosing module is open and air or exhaust gas can flow out of the exhaust tract into the dosing system.
By the partial aeration of the dosing system, a compressible air bubble forms so that when the remaining residues of the reducing agent freeze in the dosing system, the resulting ice pressure is so small that no damage occurs to the dosing system. Such a 4/2-way valve is however subject to malfunction and expensive.